/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.feilong.lib.collection4.comparators;

import java.io.Serializable;
import java.util.Comparator;

import com.feilong.lib.collection4.ComparatorUtils;

/**
 * A Comparator that will compare nulls to be either lower or higher than
 * other objects.
 *
 * @param <E>
 *            the type of objects compared by this comparator
 * @since 2.0
 */
public class NullComparator<E> implements Comparator<E>,Serializable{

    /** Serialization version. */
    private static final long           serialVersionUID = -5820772575483504339L;

    /**
     * The comparator to use when comparing two non-<code>null</code> objects.
     **/
    private final Comparator<? super E> nonNullComparator;

    /**
     * Specifies whether a <code>null</code> are compared as higher than
     * non-<code>null</code> objects.
     **/
    private final boolean               nullsAreHigh;

    //-----------------------------------------------------------------------
    /**
     * Construct an instance that sorts <code>null</code> higher than any
     * non-<code>null</code> object it is compared with. When comparing two
     * non-<code>null</code> objects, the {@link ComparableComparator} is
     * used.
     **/
    public NullComparator(){
        this(ComparatorUtils.NATURAL_COMPARATOR, true);
    }

    /**
     * Construct an instance that sorts <code>null</code> higher than any
     * non-<code>null</code> object it is compared with. When comparing two
     * non-<code>null</code> objects, the specified {@link Comparator} is
     * used.
     *
     * @param nonNullComparator
     *            the comparator to use when comparing two
     *            non-<code>null</code> objects. This argument cannot be
     *            <code>null</code>
     *
     * @throws NullPointerException
     *             if <code>nonNullComparator</code> is
     *             <code>null</code>
     **/
    public NullComparator(final Comparator<? super E> nonNullComparator){
        this(nonNullComparator, true);
    }

    /**
     * Construct an instance that sorts <code>null</code> higher or lower than
     * any non-<code>null</code> object it is compared with. When comparing
     * two non-<code>null</code> objects, the {@link ComparableComparator} is
     * used.
     *
     * @param nullsAreHigh
     *            a <code>true</code> value indicates that
     *            <code>null</code> should be compared as higher than a
     *            non-<code>null</code> object. A <code>false</code> value indicates
     *            that <code>null</code> should be compared as lower than a
     *            non-<code>null</code> object.
     **/
    public NullComparator(final boolean nullsAreHigh){
        this(ComparatorUtils.NATURAL_COMPARATOR, nullsAreHigh);
    }

    /**
     * Construct an instance that sorts <code>null</code> higher or lower than
     * any non-<code>null</code> object it is compared with. When comparing
     * two non-<code>null</code> objects, the specified {@link Comparator} is
     * used.
     *
     * @param nonNullComparator
     *            the comparator to use when comparing two
     *            non-<code>null</code> objects. This argument cannot be
     *            <code>null</code>
     *
     * @param nullsAreHigh
     *            a <code>true</code> value indicates that
     *            <code>null</code> should be compared as higher than a
     *            non-<code>null</code> object. A <code>false</code> value indicates
     *            that <code>null</code> should be compared as lower than a
     *            non-<code>null</code> object.
     *
     * @throws NullPointerException
     *             if <code>nonNullComparator</code> is
     *             <code>null</code>
     **/
    public NullComparator(final Comparator<? super E> nonNullComparator, final boolean nullsAreHigh){
        this.nonNullComparator = nonNullComparator;
        this.nullsAreHigh = nullsAreHigh;

        if (nonNullComparator == null){
            throw new NullPointerException("null nonNullComparator");
        }
    }

    //-----------------------------------------------------------------------
    /**
     * Perform a comparison between two objects. If both objects are
     * <code>null</code>, a <code>0</code> value is returned. If one object
     * is <code>null</code> and the other is not, the result is determined on
     * whether the Comparator was constructed to have nulls as higher or lower
     * than other objects. If neither object is <code>null</code>, an
     * underlying comparator specified in the constructor (or the default) is
     * used to compare the non-<code>null</code> objects.
     *
     * @param o1
     *            the first object to compare
     * @param o2
     *            the object to compare it to.
     * @return <code>-1</code> if <code>o1</code> is "lower" than (less than,
     *         before, etc.) <code>o2</code>; <code>1</code> if <code>o1</code> is
     *         "higher" than (greater than, after, etc.) <code>o2</code>; or
     *         <code>0</code> if <code>o1</code> and <code>o2</code> are equal.
     **/
    @Override
    public int compare(final E o1,final E o2){
        if (o1 == o2){
            return 0;
        }
        if (o1 == null){
            return this.nullsAreHigh ? 1 : -1;
        }
        if (o2 == null){
            return this.nullsAreHigh ? -1 : 1;
        }
        return this.nonNullComparator.compare(o1, o2);
    }

    //-----------------------------------------------------------------------
    /**
     * Implement a hash code for this comparator that is consistent with
     * {@link #equals(Object)}.
     *
     * @return a hash code for this comparator.
     **/
    @Override
    public int hashCode(){
        return (nullsAreHigh ? -1 : 1) * nonNullComparator.hashCode();
    }

    /**
     * Determines whether the specified object represents a comparator that is
     * equal to this comparator.
     *
     * @param obj
     *            the object to compare this comparator with.
     *
     * @return <code>true</code> if the specified object is a NullComparator
     *         with equivalent <code>null</code> comparison behavior
     *         (i.e. <code>null</code> high or low) and with equivalent underlying
     *         non-<code>null</code> object comparators.
     **/
    @Override
    public boolean equals(final Object obj){
        if (obj == null){
            return false;
        }
        if (obj == this){
            return true;
        }
        if (!obj.getClass().equals(this.getClass())){
            return false;
        }

        final NullComparator<?> other = (NullComparator<?>) obj;

        return this.nullsAreHigh == other.nullsAreHigh && this.nonNullComparator.equals(other.nonNullComparator);
    }

}
